In prior types of cryogenic refrigerators, a working fluid, such as helium, is introduced into a cylinder, and the fluid is expanded at one end of a piston or displacer to cool a refrigeration cylinder. In Gifford-McMahon type refrigerators a high pressure working fluid is valved into a warm end of the refrigerator, and then passes through a regenerator by movement of a displacer. The fluid, cooled in the regenerator, is then expanded at the cold end of the displacer. The movement of the displacer is driven by a rotary motor.
One stage cryogenic refrigerators and two stage cryogenic refrigerators are also known. Typically, the first stage includes a first displacer. The first displacer reciprocates the working fluid between expansion and compression. The second stage includes a second displacer. The second displacer also reciprocates the working fluid between expansion and compression. Typically, the first and second displacers are interconnected and driven by a common rotary motor.